1. Field of the Invention
The present invention relates to machines for reducing scrap plastic material and in particular to open hog rotor granulators.
2. Description of the Prior Art
Due to the high cost of virgin material it has in the plastics industry generally become the practice to recycle as much waste plastic as is possible. Such waste plastic will often include "purgings" formed by the purging of an injection molder and residual plastic remaining at the end of a production run as well as film, pipe and other rejected parts. This recycling operation is conventionally carried out in a "central" or "hog" granulator, which generally includes a central rotor with a plurality of longitudinal knives that interact with a bed knife on the wall of a cutting chamber to granulate plastic material fed into the chamber. Such hog granulators have heretofore been of two general types, open rotor and closed rotor.
An open rotor type device has a number of elongated, longitudinal pockets in the surface of the rotor. After pieces of plastic material are initially cut from larger pieces of material being fed into the cutting chamber but while such cut-off pieces are still too large to pass through a sizing screen positioned below the rotor, these cut-off pieces take up residence in these pockets while they are further reduced by the cooperative cutting action of the longitudinal knives on the rotor and the bed knife. While this open rotor design allows cooling air to dissipate heat generated by the cutting action, its disadvantage is that machines of this design will often accept into the cutting chamber pieces of plastic material to be reduced which are disadvantageously large. The principal limitation affecting the size of such pieces of plastic material to be reduced will be the size of the pockets in the rotor. Since these pockets typically run the entire length of the rotor, oversized pieces of material to be reduced will occasionally be received and take up residence in them. The fact that such large pieces of material to be reduced may be accepted into the cutting chamber requires that a relatively large motor be connected to the machine to accommodate power surges. Furthermore, a machine of an open rotor design must also be of relatively heavy construction to withstand such power surges and shock loads.
The closed rotor differs from the open rotor in that there are no pockets adjacent to the knives on the rotor surface. The surface of the rotor is effectively regular except for the knife tip projections. The advantage to this design is that power surging is minimized since the knives can only cut off material in pieces as large as the knife projections. Since, however, there are no pockets, pieces of cut-off material to be reduced which are too large to pass through the sizing screen positioned beneath the rotor will be retained between the rotor and the screen while subsequent cuts on it by the fly and bed knives are made. Due to the usually close proximity of the rotor and the screen, severe rubbing between the rotor and the retained plastic material to be reduced may occur, and large amounts of heat may be generated. The generation of this heat coupled with the introduction of only limited amounts of cooling air due to the regular surface of the rotor will sometimes result in the softening or melting of many plastic materials unless the machine's through-put rate is kept at an undesirably low level.
In view of the above described disadvantages of hog rotor granulators heretofore known, it is the object of the present invention to provide a machine of this kind which is not subject to power surges and does not require the use of a motor of unduly large capacity but which also allows heat to be effectively dissipated.